1. Field of the Invention
The present invention relates to an optical element suitable for an image pickup apparatus such as a video camera, a still video camera, or a copying machine or an image display apparatus such as a head- or face-mounted image display apparatus and, more particularly, to an optical element having a plurality of reflecting surfaces with curvatures. More specifically, the present invention relates to an optical element that allows acquisition of an accurate relative positional relationship between a plurality of reflecting surfaces having curvatures or an accurate relative positional relationship between the respective reflecting surfaces and the overall optical element, which is required to form the respective reflecting surfaces at predetermined positions, within a short period of time, and an apparatus including a holding jig for holding the optical element during a shape measurement process and an apparatus, such as an image pickup apparatus, including the optical element.
2. Related Background Art
With the recent spread of multimedia technology, not only audio/text data but also image data are handled. In general, video cameras and digital cameras are widely used to capture images.
In addition, compact cameras as phototaking means have recently been built in portable terminal apparatuses, such as portable telephones and handy computers, to allow transmission of captured image data through telephone lines immediately after a phototaking operation.
In general, the cameras of these image input apparatuses include unfocal lenses or zoom lenses made up of coaxial lenses suitable for the sizes of the respective image pickup elements.
The image pickup element of the camera of a portable terminal apparatus like the one described above is very small. For this reason, such a camera is often designed to be compact at the cost of the brightness (F-number) of the lens. In addition, there is a tendency toward a low S/N ratio. Consequently, many cameras cannot be used in dark places. Even if they can be used in dark places, images with poor image quality (low S/N ratios) are often obtained.
In addition, since many cameras include fixed focal length lenses, the field angles are always fixed. This causes inconvenience depending on the objects to be phototaken. To obviate such inconvenience, zoom lenses are mounted in these cameras. However, the use of zoom lenses inevitably increases the size of the camera. That is, the zoom lenses pose a problem in terms of portability.
With the recent decrease in the size of an image pickup element and the increase in the integration degree of a circuit system, there has been an increasing tendency toward producing a phototaking circuit unit of small size. And a decrease in the size of a camera has grown in importance.
Conventionally, this type of camera uses coaxial lenses, and hence requires a certain thickness or more in the optical axis direction. In addition, many cameras in this form have wide-angle lenses because of their application purposes. According to the arrangement of such coaxial lenses, the front-element diameter (the diameter of the front lens) is especially large.
It is therefore difficult to reduce the size of the camera as a whole, especially its profile, resulting in limitations on the degree of freedom in the form of a product and portability. This is one of the factors that interfere with the widespread use of such cameras as consumer products.
Under such circumstances, efforts have been made to develop a non-coaxial lens unit. According to this lens unit, a plurality of curved surfaces as reflecting surfaces are integrally formed on the surface of a transparent optical element. Light enters the element through a light incident surface formed by a general refracting surface and is reflected by these reflecting surfaces. Desired optical characteristics are obtained by using the reflection of these reflecting surfaces. For example, such a lens unit is disclosed in Japanese Patent Application Laid-Open No. 8-292372 (EP 0730180A). According to this reference, a plurality of transparent optical elements, each having a plurality of curved surfaces and flat surfaces as reflecting surfaces integrally formed, are used, and the relative positions of at least two optical elements of two or three or more optical elements are properly changed to zoom.
As compared with a coaxial lens unit, the front-element diameter of an optical element in this form can be reduced to reduce its thickness in the light incident direction. Therefore, development and research of such optical elements as promising techniques is urgently pursued.
An optical element of this type uses free curved surfaces (non-rotation-symmetrical aspheric surfaces). Similar to general coaxial lenses, these free curved surfaces demand high molding precisions, i.e., a shape precision of several microns and a surface precision of submicrons. Obviously, to achieve such high molding precisions, mold work must be performed with precision on the submicron order, and a shape measurement technique with precision higher than the work precision is required.
As described above, each free curved surface is formed by polishing because of the requirement of surface precision on the submicron order. To improve workability in polishing, mold pieces for the respective free curved surfaces are separately manufactured.
When, therefore, these mold pieces are assembled into a mold, positional shifts of several microns occur between the respective mold pieces. As a result, relative tilts occur between the reference surfaces forming the respective free curved surfaces.
In consideration of the surface precision required for each free curved surface, the mold must be corrected by measuring the mold or molded product with a three-dimensional measuring device having a resolution of submicrons or less and calculating the tilt amount of each free curved surface.
When the shape of an optical element is measured with this three-dimensional measuring device, measurement references must be set on a holding jig for holding the optical element. In this case, in general, measurement reference surfaces are formed on the outer surface of the holding jig, and the positions of the measurement reference surfaces are measured in a measurement process for the optical element to set reference planes and the origin of an absolute coordinate system. According to such method that sets a measurement reference for an overall optical element in reference to the measurement reference surfaces of the holding jig, the work precision of the holding jig affects the measurement data on the measurement reference surfaces, resulting in low reliability of the measurement data.
It is an object of the present invention to provide an optical element that allows accurate measurement of the shapes of the element and its surfaces which light is bound to strike, a jig for holding the optical element, and an apparatus such as an image pickup apparatus or display apparatus which includes the optical element.
According to an aspect of the present invention, there is provided an optical element comprising a surface, and a reference portion indicating a reference position of a shape of the surface, the reference portion having a curved surface for defining a point corresponding to the reference position.
According to another aspect of the present invention, there is provided an optical element comprising a surface, and a reference portion indicating a reference position of a shape of the element, the reference portion being a portion to be held when the element is built in an apparatus.
The jig of the present invention is a jig for holding the optical element when the shape of the surface of the element is to be measured. The jig includes a reference portion indicating the reference position of the jig and using one or a plurality (preferably three or more) of steel balls.
The apparatus of the present invention includes the optical element described above. This apparatus is an image pickup apparatus, such as a camera or a heador face-mounted image display apparatus.
As the surface of the optical element, a non-rotation symmetrical aspheric surface is formed as a reflecting surface on the surface of a transparent medium. This reflecting surface reflects light propagating in the transparent medium. A thin element with high optical performance can be obtained by forming a plurality of surfaces of this type.